1. Related Field
The present invention relates generally to misfire detection in an internal combustion engine, and more particularly to a Fourier-based misfire detection strategy.
2. Description of the Related Art
A misfire condition in an internal combustion engine results from either a lack of combustion of the air/fuel mixture, sometimes called a total misfire, or an instability during combustion, sometimes referred to as a partial misfire. In such case, torque production attributable to the misfiring cylinder decreases. Additionally, uncombusted fuel enters the exhaust system, which is undesirable. Because of the possible impact on the ability to meet certain emission requirements, including California Air Resource Board (CARB) emission-related requirements, engine misfire detection is needed.
Misfire detection is desired across the full speed and load operating region of a vehicle. Production algorithms have been developed and employed successfully on four and six cylinder engines since the mid-90s. In this regard, the basic strategy employed measures the period for each cylinder event (i.e., 180 degrees for a 4 cylinder and 120 degrees for a 6 cylinder) and detects misfire by monitoring the variation in the time periods. Through digital processing techniques, the reference periods can be compared against each other to determine if misfire occurred and in which cylinder(s). Even on these low number of cylinder engines (i.e., 4 and 6 cylinders), there is often the need to request exceptions from CARB due to the lack of detectability. Detectability issues can be caused by a variety of root sources. First, low load and high speed (or frequency) make detectability more difficult since the underlying misfire disturbance will have less impact on the mechanical crank system. Second, increasing the inertia of the driveline and lowering the cylinder contribution, as which occurs with higher number of cylinder engines, also compromises detectability. Engine resonance effects from crankshaft oscillations can also complicate detection. Finally, variations between different engines and the corresponding target wheels (i.e., used in producing the crankshaft-originated speed signal) also complicates detection.
Various approaches for misfire detection have been proposed in the art. For example, U.S. Pat. No. 5,487,008 entitled “METHOD AND SYSTEM FOR DETECTING THE MISFIRE OF A RECIPROCATING INTERNAL COMBUSTION ENGINE IN FREQUENCY DOMAIN” issued to Ribbens et al., disclose the use of a Discrete Fourier Transform (DFT) in the context of misfire detection. However, Ribbens et al. do not disclose strategies for effectively addressing the above-noted shortcomings in the art. Additionally, existing strategies in the art using frequency domain analysis to determine misfire have been known to use adaptive thresholds. However, such approaches still encounter the same shortcoming noted above.
There is therefore a need for a system and method to minimize or eliminate one or more of the problems as set forth above.